(1) Field of the Invention
The present invention relates to an image forming apparatus, such as a printer and a copier, that includes a fixing device, and, in particular, to technology for reducing power consumption when a heating rotating body using a resistance heating element as a heat source is used to thermally fix an unfixed image.
(2) Related Art
In recent years, in order to reduce energy and increase heating speed of a fixing device, a fixing device that thermally fixes an unfixed image formed on a recording sheet by using a heating rotating body using a resistance heating element as a heat source has been proposed as a fixing device included in an image forming apparatus such as a printer and a copier (see for example Japanese Patent Application Publication No. 2009-109997.)
Use, as the heating rotating body, of an endless heating belt including a resistance heating layer can reduce the heat capacity and a distance between the heating layer and the recording sheet, thereby increasing efficiency of heat transfer from the heating layer. As a result, power consumption of the fixing device and heating time required to warm up the fixing device can be reduced.
There are two types of the resistance heating element: a positive temperature coefficient (PTC) resistance heating element, whose electrical resistivity increases with increasing temperature, and a negative temperature coefficient (NTC) resistance heating element, whose electrical resistivity decreases with increasing temperature.
When thermal fixing of an unfixed image formed on a small-sized recording sheet is continually performed by using such a heating rotating body using a resistance heating element as a heat source, since a surface temperature in a sheet passing region of the heating rotating body (hereinafter, referred to as the “sheet passing region”) decreases upon contact with the recording sheet, power is supplied to the heating rotating body as needed to maintain the surface temperature at a target temperature at which thermal fixing is successfully performed. By the power supply, the temperature in the sheet passing region is maintained at the target temperature, but a non-sheet passing region of the heating rotating body (hereinafter, referred to as the “non-sheet passing region”) is heated to a temperature higher than the target temperature, as the heating rotating body does not contact the recording sheet in the non-sheet passing region, and thus the temperature in the non-sheet passing region does not decrease.
When the non-sheet passing region is heated to the temperature higher than the target temperature as described above, if the PTC resistance heating element is used, the electrical resistivity of the resistance heating element increases in the non-sheet passing region, and, accordingly, a power consumption rate per unit area becomes higher in the non-sheet passing region than in the sheet passing region. This results in a problem because, as an area of the non-sheet passing region increases (as a size of a recording sheet used for thermal fixing decreases), a ratio of the non-sheet passing region to the entire region including the sheet passing region and the non-sheet passing region (hereinafter, referred to as the “entire region”) increases, and the total power consumption in the entire region increases accordingly.
If the NTC resistance heating element is used, a similar problem is caused because, when the fixing device has a function to cool the non-sheet passing region by using a cooling fan and the like, and the non-sheet passing region is excessively cooled to prevent overheating in the non-sheet passing region, the power consumption rate per unit area becomes higher in the non-sheet passing region than in the sheet passing region.
In addition, if there is an extreme temperature difference between the sheet passing region and the non-sheet passing region, when the thermal fixing onto a large-sized recording sheet immediately follows the thermal fixing onto a small-sized recording sheet, a difference in image quality is caused on the large-sized recording sheet between regions corresponding to the sheet passing region and the non-sheet passing region of the small-sized recording sheet. This is problematic because a high-quality image cannot be obtained. If the temperature difference increases too much so that the electrical resistivity decreases in the non-sheet passing region, the amount of current flowing through the resistance heating element might exceed a standard value (e.g. rated current.)
Furthermore, when the NTC resistance heating element is used, if there is an extreme temperature difference between the sheet passing region and the non-sheet passing region due to overheating in the non-sheet passing region, parts of the fixing device might decrease in durability or be damaged due to heat.